The sources of noise in a vehicle include, among others, power train, driveline, tire contact patch (excited by the road surface), brakes, and wind. The noise generated by all these sources inside the vehicle's cabin covers a rather large frequency range that, for normal diesel and petrol vehicles, can go up to 6.3 kHz. Above this frequency, the acoustical power radiated by the noise sources in a vehicle is generally negligible.
It is known to use insulators, dampers and absorbers for noise attenuation in vehicles like cars and trucks to reflect and dissipate sound and thus to reduce the overall interior sound level. Such noise attenuation is typically obtained by means of a “mass-spring” system. Herein, the mass element is formed by a layer of a relatively high density impervious material, which is termed as “barrier layer,” or “heavy layer”. The spring element is formed by a layer of relatively low density material like a non-compressed felt, fleece or foam.
The stiffness of the spring is comparable to the sum of the compressibility of the air in the decoupling layer and the stiffness of its skeleton. The thickness of the noise insulation materials used is normally predetermined by the vehicle manufacturer. If this is the case, and in particular where the insulating layer has a substantially uniform thickness, changes in the resonance frequency can be carried out by increasing the mass. The function of the insulator is highly dependent on the area weight and density of the barrier layer. Typically, the insulator quality increases with increasing weight of the barrier layer. However, since the weight has a direct impact on the amount of fuel used during driving, the weight of the barrier layer is to be restricted since less weight is an economic and ecological requirement. Thus, a trend towards lower weights for the mass layer or barrier layer for classical mass-spring systems can be observed. Accordingly, recently the average area weight of a barrier layer having a thickness of from 1-4 mm has decreased from 6-7 kg/m2 to 2-3 kg/m2. Further reducing the weight of the barrier layer down to 1.5 kg/m2 or less would also reduce the insulation performance. However, with a strict requirement on low weight a balance between the acoustic performance and weight saving has to be found. For an acoustician, it is an advantage to have at disposal also a very low area weight for the barrier layer of an insulating part. An overall good acoustic performance of the vehicle would then be obtained through other measures, even though a lower level of insulation is used for some parts. Among these are, for example, a better acoustic design of the part (higher insulation of the accessories or more favourable thickness distribution), support of the insulation function with additional absorption, for an overall effect on the sound pressure level, and/or coupling of the light barrier layer to a better performing spring decoupler.
With the actual minimum area weight of around 2.5 kg/m2 for the barrier layer, there isn't any possible weight adaptation for an insulator. The corresponding minimum weight for each barrier layer foam part cannot be reduced if a lighter barrier layer is not used.
WO 2012/119654 discloses a sound attenuating trim part, comprising at least one insulating area with acoustic mass-spring characteristics comprising at least a mass layer and a decoupling layer adjacent to the mass layer, whereby the mass layer consists of a porous fibrous layer and a barrier layer. The decoupling layer and all layers are laminated together. The mass layer, i.e. the impervious barrier layer, can be made of highly filled dense materials which may include a thermoset plastic including ethylene vinyl acetate (EVA) copolymer, high density polyethylene, low polyethylene, linear low density polyethylene, polypropylene, thermoplastic elastomer or rubber, polyvinyl chloride or any combinations thereof.
WO 2012/039733 relates to filled thermoplastic polyolefin compositions useful, e.g., as sound-deadening sheeting for formed automotive applications comprising a propylene polymer having a density equal to or greater than 0.885 g/cm3, one or more linear ethylene polymer(s) and/or substantially linear polymer(s), a plasticizer, and a filler. Comparison example C* of this reference relates to a composition containing no propylene polymer but a combination of a substantially linear ethylene-octene copolymer having a density of 0.875 g/cm3 and a melt flow index of 3 g/10 min at 190° C. under a load of 2.16 kg and a low density polyethylene having a density of 0.921 g/cm3 and a melt flow index of 0.22 g/10 min at 190° C. under a load of 2.16 kg. Thermoformed articles prepared from this composition and which are stacked do not exhibit a sufficient stiffness in order to be shipped.
U.S. Pat. No. 4,438,228 relates to filled thermoplastic compositions useful, e.g., as sound-deadening sheeting for automotive carpet. The compositions are obtained by blending about 5-55% by weight of ethylene/α-olefin copolymer, the α-olefin containing from 4 to 10 carbon atoms, about 2-12% by weight of plasticizer selected from the group consisting of processing oils, epoxidized oils, polyesters, polyethers and polyether esters, about 40-90% by weight of filler, and optionally elastomeric polymers. Replacement of the ethylene/α-olefin copolymer by low-density polyethylene homo-polymer results in products having reduced stiffness and which are not suitable for the intended use. The same applies when part of the ethylene/α-olefin copolymer is replaced by an elastomeric polymer such as ethylene-propylene-diene monomer (EPDM) rubber.